As is known in the art, there has been a trend to develop materials at the nanoscale level. Manufacturing of nanoscale level devices is a challenge that needs to be addressed before the potential of nanotechnology becomes a reality. Manufacturing is defined as the transformation of materials and information into goods for the satisfaction of customer needs. Conventional manufacturing strives to produce goods in large volume with high quality, fast production rate, low cost and reasonable flexibility to accommodate the varying requirements of the customers. These attributes are equally applicable to the manufacturing of nanoscale products.
One type of nanoscale product comprises carbon nanotubes (CNTs). A carbon nanotube can be thought of as a hexagonal network of carbon atoms that has been rolled up to make a seamless cylinder. The cylinder can be tens of microns long, and each end is “capped” with half of a fullerene molecule. Single-wall nanotubes (SWNTs) can be thought of as the fundamental cylindrical structure, and these SWNTs form the building blocks of both multi-wall nanotubes and ordered arrays of single-wall nanotubes called ropes.
One method of forming a carbon nanotube comprises taking a sheet of graphite and reducing the size of the sheet such that the sheet becomes an extremely narrow strip of material. At a width of approximately 30 nanometers the strip curls about a lengthwise axis and the opposing carbon bonds at the side edges of the strip join to form a tube approximately 10 nanometers in diameter. Thinner tubes having a diameter of between 10 nanometers and 5 nanometers can be formed in the same manner. It is also possible to produce multiwall carbon nanotubes (MWNTs) by curving a number of sheets of graphite (typically 3 to 8 sheets) in a similar manner as forming a single wall carbon nanotube.
CNTs may also be prepared by laser vaporization of a carbon target in a furnace at approximately 1200° C. A cobalt-nickel catalyst helps the growth of the nanotubes because the catalyst prevents the ends of the CNTs from being “capped” during synthesis, and about 70-90% of the carbon target can be converted to single-wall nanotubes. While multi-wall carbon nanotubes do not need a catalyst for growth, single-wall nanotubes are preferably grown with a catalyst.
A carbon-arc method to grow arrays of SWNTs has also been developed. In this method, ordered nanotubes are produced from ionized carbon plasma, and joule heating from the discharge generated the plasma. In a scanning electron microscope (SEM), the nanotube material produced by either of these methods looks like a mat of carbon ropes. The ropes are between 10 and 20 nm across and up to 100 μm long. When examined in a transmission electron microscope (TEM), each rope is found to be comprised of a bundle of single-wall carbon nanotubes aligned along a single direction. X-ray diffraction, which views many ropes at once, shows that the diameters of the single-wall nanotubes have a narrow distribution with a strong peak.
The unique electronic properties of carbon nanotubes are due to the quantum confinement of electrons in a direction which is normal to the direction of a central longitudinal axis of the nanotube. In the radial direction, electrons are confined by the monolayer thickness of the graphite sheet. Around the circumference of the nanotube, periodic boundary conditions come into play. Because of this quantum confinement, electrons can only propagate along the nanotube axis, and so their wavevectors point in this direction. The resulting number of one-dimensional conduction and valence bands effectively depends upon the standing waves that are set up around the circumference of the nanotube.
The density of electronic states as a function of energy has been calculated for a variety of nanotubes. While conventional metals have a relatively smooth density of states, nanotubes are characterized by a number of singularities, where each peak corresponds to a single quantum subband. These singularities are important when interpreting experimental results, such as measurements obtained from scanning tunneling spectroscopy and resonant Raman spectra, the two techniques that have contributed the most to understanding the one-dimensionial properties of nanotubes.
A nanotube may be either metallic or semiconducting, however the chemical bonding between the carbon atoms is the same in both cases. This is due to the very special electronic structure of a two-dimensional graphite sheet, which is a semiconductor with a zero band gap. In this case, the top of the valence band has the same energy as the bottom of the conduction band, and this energy equals the Fermi energy for one special wavevector, the so-called K-point of the two-dimensional Brillouin zone (i.e. the corner point of the hexagonal unit cell in reciprocal space). A nanotube becomes metallic when one of the few allowed wavevectors in the circumferential direction passes through this K-point.
As the nanotube diameter increases, more wavevectors are allowed in the circumferential direction. Since the band gap in semiconducting nanotubes is inversely proportional to the tube diameter, the band gap approaches zero at large diameters, just as for a graphene sheet. At a nanotube diameter of about 3 nm, the band gap becomes comparable to thermal energies at room temperature.
Calculations show that concentric pairs of metal-semiconductor and semiconductor-metal nanotubes are stable. Nanometer-scale devices could therefore be based on two concentric nanotubes or the junction between nanotubes. For example, a metallic inner tube surrounded by a larger semiconducting (or insulating) nanotube would form a shielded cable at the nanometer scale. One might then envisage nanoscale electronic devices made completely from carbon that would combine the properties of metals and semiconductors, without the need for doping.
Since nanotubes are typically a few microns long, electrical contacts can be made by modern lithographic techniques. Single-wall carbon nanotubes thus provide a unique system for studying single-molecule transistor effects, in which an electrode close to the conducting nanotube is used to modulate the conductance. Another area of research is focused on the mechanical properties of carbon nanotubes. By analogy to graphite and carbon fibers, nanotubes are very strong and have high elastic moduli. Single-wall carbon nanotubes are also very strong and resist fracture under extension, just as the carbon fibers commonly used in aerospace applications. A nanotube can be elongated by several percent before it fractures. Unlike carbon fibers, however, single-wall nanotubes are remarkably flexible. They can be twisted, flattened and bent into small circles or around sharp bends without breaking, and severe distortions to the cross-section of nanotubes do not cause them to break.
Another advantage of nanotubes is their behavior under compression. Unlike carbon fibers, which fracture easily under compression, carbon nanotubes form kink-like ridges that can relax elastically when the stress is released. As a result, nanotubes not only have the desirable properties of carbon fibers, but are also much more flexible and can be compressed without fracture. The mechanical properties of carbon nanotubes would make them ideal for manipulating other nanoscale structures. Many of the applications now being considered involve multi-wall nanotubes, partly because they have been available for much longer, and partly because many of these applications do not explicitly depend on the one-dimensional quantum effects found mainly in single-wall nanotubes.
In the same way that carbon fibers are used in composites to strengthen a structure or to enhance the electrical conductivity of the main constituent, carbon nanotubes can be combined with a host polymer (or metal) to tailor their physical properties to specific applications. Since carbon nanotubes are so small, they can be used in polymer composites that are formed into specific shapes, or in a low-viscosity composite that is sprayed onto a surface as a conducting paint or coating.
Carbon nanotubes could also be used in displays or for the tips of electron probes. Other applications could result from the fact that carbon nanotubes can retain relatively high gas pressures within their hollow cores.
Several efforts have been made to grow nanotubes into patterned configurations (e.g., arrays), however the resulting arrays typically include several nanotubes that are irregularly spaced and have varying heights. Other efforts have shown that the growth of aligned nanotubes in several directions can be controlled in a single process. Though the functional feasibility to integrate nanostructures into micro-devices has been demonstrated by growing them into patterns, process couplings and scale mismatches between nano- and micro-fabrication processes limit the practical production of integrated devices. It has been stated that growing a uniform length nanotube-tip-array on 1 cm2 area would be equivalent to growing a perfectly healthy and uniform length grass lawn on 1,000 acres.
There have been few efforts regarding the handling and manufacturing of carbon nanotubes. One group has worked on the directed assembly of one-dimensional nanostructures into functional networks by fluidic assembly with a surface-patterning technique. Another research group developed a method of assembling single-walled CNTs into long ribbons and fibers. In this method the nanotubes are dispersed in surfactant solution and then the nanotubes are recondensed in the flow of a polymeric solution to form a nanotube fiber. Companies are selling commercial quantities of nanotubes in the form of soot, which cannot be handled effectively during the subsequent manufacturing processes. Other companies are working on flat panel displays using carbon nanotubes as emission tips. None of theses companies have reported commercially viable manufacturing solutions for the mass production of nanotubes.
As described above, there exists several potential applications using nanotubes, however widescale use of nanotubes will only become feasible if massive production of carbon nanotubes becomes a reality.